U.S. patent application number 11/860139 was filed with the patent office on 2008-07-31 for image forming apparatus.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to Takeshi Tomizawa.
Application Number | 20080181647 11/860139 |
Document ID | / |
Family ID | 38951736 |
Filed Date | 2008-07-31 |
United States Patent
Application |
20080181647 |
Kind Code |
A1 |
Tomizawa; Takeshi |
July 31, 2008 |
IMAGE FORMING APPARATUS
Abstract
An image forming apparatus comprising: an image bearing member;
a toner image forming means forming a toner image on the image
bearing member, using light toners, and dark toners which have a
same hue as that of the light toners and are darker than the light
toners; a transfer means which electrostatically transfers a toner
image on the image bearing member onto a recording material; and an
adjusting means which adjusts the toner image forming means so that
a rate between the dark toners and the light toners included in a
toner image with predetermined density on the image bearing member
is changed according to the roughness of a surface of the recording
material onto which the toner image on the image bearing member is
transferred.
Inventors: |
Tomizawa; Takeshi;
(Abiko-shi, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
38951736 |
Appl. No.: |
11/860139 |
Filed: |
September 24, 2007 |
Current U.S.
Class: |
399/74 |
Current CPC
Class: |
G03G 9/09 20130101; G03G
15/5029 20130101; G03G 15/0126 20130101; G03G 9/0821 20130101; H04N
1/295 20130101; G03G 2215/0119 20130101; G03G 9/0926 20130101; H04N
1/2323 20130101; G03G 9/0823 20130101 |
Class at
Publication: |
399/74 |
International
Class: |
G03G 15/00 20060101
G03G015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 29, 2006 |
JP |
2006-268291 |
Claims
1. An image forming apparatus comprising: an image bearing member;
toner image forming means for forming a toner image on the image
bearing member, using light toners, and dark toners which have a
same hue as that of the light toners and are darker than the light
toners; transfer means for electrostatically transferring a toner
image on the image bearing member onto a recording material; and
adjusting means for adjusting the toner image forming means so that
a rate between the dark toners and the light toners included in a
toner image with predetermined density on the image bearing member
is changed according to a roughness of a surface of the recording
material onto which the toner image on the image bearing member is
transferred.
2. The image forming apparatus according to claim 1, wherein the
adjusting means adjusts the rate so that the density of a toner
image with the lowest density among toner images formed by using
the light toners and the dark toners when the surface is coarse is
lower than that when the surface is smooth.
3. The image forming apparatus according to claim 2, further
comprising resolution changing means for adjusting the toner image
forming means so that the resolution of a toner image formed by the
toner image forming means when the surface is coarse is lower than
that when the surface is smooth.
4. The image forming apparatus according to claim 3, further
comprising detecting means for detecting the surface of the
recording material.
5. The image forming apparatus according to claim 2, further
comprising input means for inputting information on the roughness
of a surface onto which the toner image on the recording material
is transferred.
6. An image forming method comprising the steps of: forming a toner
image on an image bearing member, using light toners and dark
toners which have a same hue as that of the light toners, and a
higher density than that of the light toners; electrostatically
transferring the toner image on the image bearing member onto a
recording material; and adjusting the toner image forming means so
that a rate between the dark toners and the light toners included
in a toner image with a predetermined density on the image bearing
member is changed according to a roughness of a surface of the
recording material onto which the toner image on the image bearing
member is transferred.
7. The image forming method according to claim 6, wherein the rate
is adjusted so that, when the surface is coarse, the density of a
toner image with the lowest density among toner images using the
light toners and the dark toners is lower than that when the
surface is smooth.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an electrographic image
forming apparatus (hereinafter, simply called "image forming
apparatus), such as a copying machine, a printer, facsimile
terminal equipment, and a multifunction printer, which forms images
using an electrophotographic system. Especially, the present
invention relates to an electrophotographic image forming apparatus
using dark toners and light toners, which have a same hue, and are
different from each other in density.
[0003] 2. Description of the Related Art
[0004] The image forming apparatus forming color images forms four
color toner images on, for example, an photosensitive drum of an
image bearing member, wherein the four colors include Y (yellow), M
(magenta), C (cyan), and K (black). The above toner images are
sequentially superimposed and transferred on, for example, a sheet
(transfer material) held on a transfer drum (transfer film). In the
above case, an electrostatic latent image of, for example, cyan as
a first color is formed on a photosensitive drum, based on input
signals including read image information, and then a C toner image
obtained by developing the cyan latent image is transferred on the
sheet on the transfer drum. The above series of transfer processes
are sequentially repeated for other three colors of Y toners, M
toners, and K toners as a second color, a third color, and a fourth
color in this order, thereby a color image is obtained.
[0005] Recently, latent images are collected and formed on the drum
surface of the photosensitive drum, wherein the drum surface bears
dots of a predetermined potential, and a solid portion, a half tone
portion, and a line portion are expressed by changing the density
of the dots in the image forming apparatus using digital image
signals. However, in the above case, toner particles are hardly
placed on the dots, and there is caused a state in which the toner
particles deviate from the dots. Thereby, it is difficult to obtain
the gradations of a toner image corresponding to the dot density
ratio between the black portions and the white portions of a
digital latent image. Moreover, when resolution is improved by
smaller dots in order to improve image quality, it becomes further
difficult to reproduce a latent image formed by a collection of
microdots. Especially, the resolution and the gradations in
highlight portions are deteriorated to cause a tendency to loose
sharpness in the color of the image. Moreover, disturbance of the
dots causes a sense of granularity which leads to reduction in the
image quality for the highlight portions, and image unevenness by
the granularity is an unpredictable unstable element of the image
quality.
[0006] On the other hand, an inkjet recording method is a simple
system as can be seen in a technology processing dark-colored ink
and light-colored ink, which has been disclosed in, for example,
Japanese Patent Application Laid-Open No. 58-039468. Moreover, the
inkjet recording method has been supported by use of recent and
excellent high-quality specialized-paper and does not cause the
above-described problems raised by the electrophotographic system.
Furthermore, the inkjet recording method has a unique advantage
that there is caused no sense of granularity, because the method
used dark-colored ink and light-colored ink. As excellent
performances are obtained by use of, especially, light-colored ink,
an electrophotographic system applying light-colored ink would
result in a largely improved system.
[0007] Furthermore, even with regard to optical dot gain which is a
barrier for developing an electrophotographic system forming a
high-quality image, an idea of introducing light-colored toners is
effective for solving the problems caused by use of micro toners.
Based on the above idea, there has been proposed an image forming
apparatus (refer to, for example, Japanese Patent Application
Laid-Open Nos. 11-84764 and 2000-305339) forming an image by
combining a plurality of toners which are different from one
another in density, for example, by using light-colored toners
(light toners) in the highlight portions, and dark-colored toners
(dark toners) in a solid portions. Moreover, there has been
proposed an image forming apparatus (refer to, for example,
Japanese Patent Application Laid-Open No. 2000-347476) in which
dark toners and light toners having a maximum reflection density
below half of the maximum reflection density of each of the dark
toners are combined. Moreover, there has been proposed an image
forming apparatus (refer to, for example, Japanese Patent
Application Laid-Open No. 2000-231276) having a configuration in
which dark toners with an image density of 1.0 or more and light
toners with an image density less than 1.0 are combined, when toner
quantity on a sheet is 0.5 mg/cm.sup.2. Furthermore, there has been
proposed an image forming apparatus (refer to, for example,
Japanese Patent Application Laid-Open No. 2001-290319) in which
toners are combined so that a recording density ratio between the
dark toner and the light toner is adjusted at 0.2 through 0.5.
[0008] In the image forming apparatuses disclosed in the
above-described patent documents, it has been assumed that plain
paper is mainly used as a kind for a sheet as transfer material. As
described above, the inkjet recording method has realized a
high-level image forming technology by using high-quality
specialized paper with excellent printing performances based on
stable high-resolution binary recording. It is very difficult to
introduce the image forming technology as it is based on the above
inkjet recording method into the electrophotographic system which
has mainly used plain paper. Conventionally, the
electrophotographic system has improved density gradations by using
a low resolution screen which has been used for printing, in order
to reduce performance difference between high quality specialized
paper and plain paper. Thereby, problems, such as coarseness and
granularity, peculiar to the electrophotographic system may be
solved by using light toners in low density portions.
[0009] Incidentally, greater importance is often placed on
smoothness in highlight portions as an image using dark toners and
light toners when the quality of an image like a picture is
improved.
[0010] However, toner quantity placed in highlight portions is
increased as a result of smoothing the highlight portions.
Moreover, when a toner image is formed on a sheet (recording
material) with a rough surface, transfer unevenness of light toners
in highlight portions is easily caused in a toner image with an
increased quantity of toner.
SUMMARY OF THE INVENTION
[0011] An object of the invention is to provide an image forming
apparatus using dark toners and light toners, which have a same
hue, and are different from each other in density, wherein,
according to the image forming apparatus, a high-quality image may
be formed, independent from the surface roughness of a recording
material.
[0012] Moreover, another object is to provide an image forming
apparatus including: an image bearing member; a toner image forming
means forming a toner image on the image bearing member, using
light toners, and dark toners which have a same hue as that of the
light toners and are darker than the light toners; a transfer means
which electrostatically transfers a toner image on the image
bearing member onto a recording material; and an adjusting means
which adjusts the toner image forming means so that a rate between
the dark toners and the light toners included in a toner image with
predetermined density on the image bearing member is changed
according to the roughness of a surface onto which the toner image
on the recording member is transferred.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a view showing an image forming apparatus
according to an embodiment of the invention;
[0014] FIG. 2 is a view showing a structure of an image reading and
detecting portion;
[0015] FIG. 3A is a schematic view showing an image formed on
recording paper A as an example of sheet types according to a first
embodiment;
[0016] FIG. 3B is a schematic view showing an image formed on
recording paper B as an example of sheet types according to a first
embodiment;
[0017] FIG. 3C is a schematic view showing an image formed on
recording paper C as an example of sheet types according to a first
embodiment;
[0018] FIG. 4A is a schematic view showing an image formed on the
recording paper A after pixel data conversion;
[0019] FIG. 4B is a schematic view showing an image formed on the
recording paper B after pixel data conversion;
[0020] FIG. 4C is a schematic view showing an image formed on the
recording paper C after pixel data conversion;
[0021] FIG. 5 is a flow diagram showing a series of operations of
from image processing to sheet type decision;
[0022] FIG. 6 is a schematic view showing thresholds according to
which pieces of recording paper A, B, and C are decided;
[0023] FIG. 7 is a functional diagram showing a configuration;
[0024] FIG. 8 is a matrix of basic three elementary colors RGB;
[0025] FIG. 9 shows characteristic curves representing recording
rates of dark toners and light toners;
[0026] FIG. 10 is characteristic curves determining recording rates
of dark and light toners;
[0027] FIG. 11 is a view showing an operation panel;
[0028] FIG. 12A is a view showing image unevenness in a case a
processing for reducing a line number is not performed before and
after the toner image is transferred from a photosensitive drum to
a sheet in a second embodiment according to the invention; and
[0029] FIG. 12B is a view showing image unevenness in a case a
processing for reducing a line number is not performed before and
after the toner image is transferred from the photosensitive drum
to a sheet in the second embodiment according to the invention.
DESCRIPTION OF THE EMBODIMENTS
[0030] Hereinafter, an electrophotographic full-color image forming
apparatus, and an image adjusting method will be described as an
image forming apparatus according to an exemplary embodiment of the
invention in detail, referring to drawings.
[0031] As shown in FIG. 1, the electrophotographic full-color image
forming apparatus according to the present embodiment has a digital
color image reading portion (hereinafter, simply called "reading
portion") 1R in the upper portion, and a digital color image
printing portion (hereinafter, simply called "printing portion") 1P
in the lower portion. In the above case, the printing portion 1P
may operate based on a read signal output from the reading portion
1R.
[0032] In the reading portion 1R, the light of an exposure lamp 32
is applied for exposure scanning to an original 30 mounted on an
original base plate glass 31, and a light image reflected from the
original 30 is focused onto a full-color CCD sensor 34 through a
lens 33 to obtain color-separated color image signals. The
color-separated color image signals pass through a not-shown
amplification circuit for image processing in a video processing
means, and are sent to the printing portion 1P through an image
memory. An image signal from a computer, an image signal based on
facsimile communication, and the like are input to the printing
portion 1P as well as the read signal of an image which is output
from the reading portion 1R are input to the printing portion
1P.
[0033] In the printing portion 1P, for example, six photosensitive
drums 1a through 1f as an image bearing member are supported in the
direction of arrows shown in the drawing. Hereinafter, the six
photosensitive drums 1a through 1f are represented by a symbol "1".
Similarly, other materials are described with a representative
symbol. A pre-exposure lamp 11, a corona primary charger 2, a laser
exposure optical system 3, a potential sensor 12, six development
devices 40 loading toners which are different from one another in
spectral characteristics, a transfer device 5A, and a cleaning
device 6 are arranged around the photosensitive drum 1. While the
above devices form six image forming portions (toner image forming
means) Pa through Pf, obviously, the number of the portions is not
limited to six but image forming portions of an arbitrary number of
four or more may be provided.
[0034] Incidentally, a light-colored magenta (LM) toner is loaded
in a development device 41 as one of the six development devices 40
and a light-colored cyan (LC) toner is loaded in a development
device 42. Furthermore, there are loaded a yellow (Y) toner in a
development device 43, a magenta (M) toner in a development device
44, a cyan (C) toner in a development device 45, and a black (K)
toner in a development device 46. Each of the above toners is
charged negative polarity in the corresponding development
device.
[0035] In order to reduce the granularity, the magenta (M) toner
loaded in the development device 44 uses a dark-colored M toner
(hereinafter, called "dark toner") and a light-colored M toner
(hereinafter, called "light toner"). Similarly, a dark toner and a
light toner are also used for the cyan (C) toner loaded in the
development device 45.
[0036] The dark magenta toner and the light magenta toner are
simultaneously used in one image, and the dark magenta toner and
the light dark magenta toner are superimposed in the same pixel
forming the image. The same holds true for the dark and the light
cyan toners.
[0037] Furthermore, it is also possible to provide an image forming
portion having a development device loading toners of metal such as
gold and silver, or toners including a fluorescent agent. Moreover,
technical ideas of the invention is achieved even when a
single-component developing agent including only toners is used,
though a two-component developing agent using a combination of
toners and carriers is loaded in the development device 40.
[0038] Moreover, a not-shown laser output portion converts the read
image signal output from the reading portion 1R into an optical
signal, and laser light converted into the optical signal is
reflected on a polygon mirror in the scanner 3 in the laser
exposure optical system. Reflected laser light is fully projected
onto the drum surfaces of the six photosensitive drums 1 through
the lens and the reflection mirrors.
[0039] According to the above-described configuration, images are
formed as follows in the printing portion 1P.
[0040] The photosensitive drum 1 is rotated in the direction of the
arrow, and the photosensitive drum 1 after diselectrification by
the pre-exposure lamp 11 is uniformly charged by the primary
charger 2 to form an electrostatic latent image on the
photosensitive drum 1 by exposure for each of separated colors.
[0041] Subsequently, the development device 40 is operated to
develop a latent image on the photosensitive drum 1, and a visible
image (toner image) including resins and pigments as a base body is
formed on the photosensitive drum 1. The toners in the development
device 40 are supplied from accommodation portions (hoppers) 60 for
each color which are adjacent to the corresponding scanner 3 at any
desired timing, so that the toner ratios (or toner quantities) in
the development device 40 are kept constant. In each of the
transfer devices 5, the toner images formed on the photosensitive
drums 1 are primarily transferred onto a corresponding intermediate
transfer belt 5 as an intermediate transfer body, and each of the
toner images are sequentially superimposed on the above transfer
belt 5.
[0042] The intermediate transfer belt (the image bearing member,
the intermediate transfer body) 5 is wound around a driving roller
51, and driven rollers 52 and 54, and transmits rotary power from a
rotary driving source to the driving roller 51. The driving roller
51 is rotationally driven to cause rotational travelling of the
intermediate transfer belt 5. A transfer cleaning device 50 is
arranged in a side opposite to the driving roller 51 with the
intermediate transfer belt 5 existing therebetween. Moreover, the
transfer cleaning device 50 can be contacted with and separated
from the driving roller 51. The transfer cleaning device 50 is
pressed onto the driving roller 51 after superimposing images for
required colors on the intermediate transfer belt 5, and then
remaining toners on the intermediate transfer belt 5 are cleaned
and removed after toner images are transferred onto a sheet as a
transfer material (recording material).
[0043] Sheets are conveyed one by one from a storage portion 71,
72, or 73, or a manual paper feed portion 74 through a
corresponding one of paper feed means 81 through 84. A sheet the
skew of which is corrected by a registration roller 85 is sent to a
secondary transfer portion T at desired timing, a voltage of
positive polarity is applied to a secondary transfer roller
(transfer means) 56 from a transfer power supply 561, and a toner
image on the intermediate transfer belt 5 is transferred to the
sheet. The sheet on which the toner image has been transferred in
the secondary transfer portion 56 passes through a conveying
portion 86, the toner image is fixed on the sheet at a heat-roller
fixing device 9, and the sheet is ejected to an output tray or a
postprocessing device. On the other hand, remaining toners after
transferring are cleaned by the transfer cleaning device 50 after
secondary transferring, and then the intermediate transfer belt 5
serves for primary transfer processing again in the image forming
portions.
[0044] Moreover, when an image is formed on the both sides of a
sheet, that is, in the case of double-sided printing, a
conveying-path guide 91 is driven just after the sheet passes
through the fixing device 9. Once the sheet is led to a reversing
passing 76 through a conveying path 75, a reversing roller 87 is
reversely rotated and the sheet is receded in an opposite direction
to the direction the sheet has been sent in a state in which the
rear end of the sent sheet is at the head, and then the sheet is
sent to a double-sided conveying path 77. The sheet passing through
the double-sided conveying path 77 undergoes skew correction and
timing adjustment by a double-sided conveying roller 88, and is
conveyed to the registration roller 85 at desired timing.
Subsequently, the above-described image forming process is executed
again for image transferring onto one side.
[0045] Then, image forming in each of image forming modes will be
described.
[0046] As described above, dark toners and light toners, which have
a same hue, are prepared for magenta (M) toners and cyan (C)
toners. Materials which have a same hue means color developing
components (pigments) having a same spectral characteristic as one
another. But, the word "same" does not always require strict
identity, and is generally allowed to include a range within which
colors may be regarded as a same color as one another according to
a usual concept on, for example, four colors such as Y, M, C, and
K. Moreover, toners which are belonging to a same hue and have
different densities from one another usually means toners in which
color developing components (pigments) included in the toners
having resins and the color developing components as a base body
are the same as one another in the spectral characteristic and the
quantity is different form one another.
[0047] Hereinafter, a light toner will be defined. The light toners
are defined to belong to a same hue, and to be a toner with
relatively low density among a combination of several kinds of
toners with different densities from one another. In the present
embodiment, light-colored light toners with low density has optical
density less than 1.0 after fixing for a toner quantity of 0.5
mg/cm.sup.2 on a sheet. On the other hand, dark-colored dark toners
with high density has optical density of 1.0 or more after fixing
for a toner quantity of 0.5 mg/cm.sup.2 on a sheet. In such a case,
the pigment quantity of a dark toner is adjusted so that optical
density after fixing is 1.6 at a placed toner quantity of 0.5
mg/cm.sup.2 on a sheet. Moreover, a light toner is designed so that
optical density after fixing is 0.8 at a placed toner quantity of
0.5 mg/cm.sup.2. Thus, two kinds of dark toners and light toners
are appropriately mixed in quantity for magenta and cyan to obtain
important tone reproduction as a target for cyan and magenta.
[0048] As shown in FIG. 2, a sheet reading and detecting portion
(recording material detecting means) 123 is provided in the
full-color image forming apparatus according to the present
embodiment. In the sheet reading and detecting portion 123, light
is applied onto the surface of a sheet 32 conveyed from a paper
cassette 102 by a paper feeding roller 103, and reflected light is
focused for image forming to detect a specific area in the sheet
32. The sheet reading and detecting portion 123 has an LED 33 as a
light applying means, a CMOS sensor 34 as a reading means, lenses
35 and 36 as an image forming means, and the like.
[0049] Accordingly, light emitted from the LED 33 as a light source
is applied onto the surface of the sheet conveying guide 31, or the
surface of the sheet 32 on the sheet conveying guide 31 through the
lens 35. Reflected light from the sheet 32 is focused through the
lens 36 to form an image in the CMOS sensor 34. The image formed
above is detected by the CMOS sensor 34 to read the surface of the
sheet conveying guide 31 or the sheet 32. Here, light from the LED
33 is arranged so that the light is applied at a predetermined
angle with respect to the surface of the sheet 32 in a slanting
direction.
[0050] FIG. 3A through FIG. 3C are views showing an example in
which the surfaces of three kinds of sheets 32 (hereinafter, called
"recording paper A, B, and C") are read by the CMOS sensor 34 in
the sheet reading and detecting portion 123 to perform digital
processing of the output from the CMOS sensor 34 for the surface of
each of the recording paper A, B, and C in 8 pixels.times.8 pixels.
The above digital processing is performed by conversion of an
analog output from the CMOS sensor 34 into pixel data of eight
bits, using a not-shown analog-to-digital converter as a converting
means.
[0051] FIG. 3A is a schematic view showing the surface of the
recording paper A such as so-called rough paper having relatively
coarse cellulose on the sheet surface. Similarly, FIG. 3B is a
schematic view showing the surface of the recording paper B such as
so-called plain paper which has been usually used. Furthermore,
FIG. 3C is a schematic view showing the surface of the recording
paper C such as smooth paper (gross paper) having fully compressed
cellulose. Pictures 43, 44, and 45 shown in FIG. 4A through FIG.
4C, respectively, are obtained by digital processing of each of the
pictures on the corresponding surface read by detection using the
CMOS sensor 34. As shown in the drawing, the pictures obtained by
reading the surfaces are different from one another, depending on
the materials and the kinds of the sheets such as the recording
paper A, B, and C. Those phenomena are caused by a fact that the
cellulose states on the paper surface are different from one
another. That is, decision of the surface state of paper cellulose
may be made by a picture obtained by digital processing based on
detecting by the CMOS sensor 34 for reading.
[0052] Reading operation for the surfaces of the recording paper A,
B, and C will be described, referring to a flow diagram shown in
FIG. 5.
[0053] In the first place, the CMOS sensor 34 reads the surface at
several locations on the sheet 32, that is, the recording paper A,
B, and C over several times at steps S50 and S51. Subsequently,
constants for gain operation and filter operation in a not-shown
filter operation means are adjusted (step S53) after turning off
the LED (step S52). The gain operation and the filter operation are
programmably processed by a control processor. The gain operation
is performed by adjusting, for example, the gain of an analog
output from the CMOS sensor 34. That is, as the surface of a sheet
may not be read well when the quantity of light reflected on the
sheet surface is too much or too little, the gain is adjusted when
a signal change may not be led. Moreover, with regard to the filter
operation, operations based on, for example, a 1/32 frequency
division, a 1/16 frequency division, and a 1/4 frequency division
are performed to remove a noise component output from the CMOS
sensor 34 when, for example, digital data of 256 gradations by
eight bits is obtained after analog to digital conversion of an
analog output from the CMOS sensor 34.
[0054] Then, it is determined (step S54) whether adjustment of the
above filter and the above gain has been completed to the extent
that it is possible to decide that a sheet is corresponding to
which of the recording materials A, B, and C.
[0055] When it is determined that the adjustment of the filter and
the gain is completed to the extent that it is possible to decide
the type of the recording material for the sheet (Yes), comparison
operation of surface information is performed (step S55). When it
is determined that it is not adjusted to the extent that it is
impossible to decide the type of the recording material for the
sheet (No), reading of the surface of the sheet is tried again,
returning to the step S51. A sheet kind is decided, based on the
result of the above surface-information comparison operation (step
S56), and then there is decided an image processing method
according to the surface roughness of the sheet (step S57).
[0056] According to a technique for the surface-information
comparison operation, pixel highest density Dmax, and a pixel
lowest density Dmin are led from a result obtained by reading the
sheet surface at several regions. The above operations are executed
for each of read regions to perform averaging processing of the
obtained pixels. On a sheet like the recording paper A shown in
FIG. 3 and FIG. 4, a lot of shadows of cellulose are generated when
there are coarse paper cellulose on the sheet surface. Thereby,
there is caused a greater density difference between a bright
location and a dark location to make the difference between highest
pixel density Dmax and lowest pixel density Dmin greater.
Incidentally, a difference between Dmax and Dmin becomes smaller
because there are a few shadows of cellulose on the surface of a
sheet like the recording paper C shown in FIG. 3 and FIG. 4. The
above-described comparison operation is performed to decide the
paper kind, that is, the surface roughness of a sheet.
[0057] FIG. 6 is a schematic view showing a technique for deciding
the paper kinds of the recording paper A, B, and C according to
subtracted values of Dmax-Dmin. In the above case, thresholds used
as a standard for deciding paper kinds, for example, X and Y are
stored in a nonvolatile memory in a DC controller beforehand.
However, the thresholds are not limited to the above two values
such as X and Y, and may be set at two or more values. The number
of sheets, that is, three types of the recording paper A, B, and C
are only an example for deciding the paper kind.
[0058] Accordingly, the sheet is decided as the recording paper A
when the value of Dmax-Dmin is larger than the threshold Y, that
is, Dmax-Dmin>Y. Similarly, the sheet is decided as the
recording paper B when the value of Dmax-Dmin is smaller than the
threshold Y, and larger than the threshold X, and the sheet is
decided as the recording paper C when the value of Dmax-Dmin is
smaller than the threshold X.
[0059] Then, as shown in a functional block diagram shown in FIG.
7, an image signal output from full-color sensor 100 is input to an
analog signal processing portion 101 for adjustment of a gain and
an offset. After the adjustment, the image signals are converted
into RGB digital signals of, for example, eight bits (0 through 255
levels: 256 gradations) in an analog to digital converting portion
102 for each of color components. In a shading correction portion
103, a signal reading a standard white board (not-shown) is used
for each color, and gains are optimized, corresponding to each of
the CCD sensor cells, for shading correction in order to eliminate
dispersions in sensitivity for each of cells in a sensor cell group
including arranged CCDs.
[0060] A line delay portion 104 corrects a spatial displacement
included in the image signal output from the shading correction
portion 103. The above spatial displacement is caused because each
of the line sensors in the full-color sensor 100 are arranged
separated at a predetermined distance from one another in the
sub-scanning direction. Specifically, line delay of each of color
component signals of R (red) and of G (green) is performed in the
sub-scanning direction with reference to a B (blue) color component
signal, and the phases of the three color component signals are
synchronized.
[0061] An input masking portion 105 converts a color space of the
image signal output from the line delay portion 104 into an NTSC
standard color space, using a matrix operation expression shown in
FIG. 8. That is, the color space of each color component signal
output from the full-color sensor 100 is converted into the NTSC
standard color space, though the color space of each color
component signal is decided by the spectral characteristics of the
filters for each color component.
[0062] A lookup table (LUT) including, for example, ROMs and the
like forms a LOG converting portion 106 converting an RGB
brightness signal output from the input masking portion 105 into a
CMY density signal. A line delay memory 107 delays the image signal
output from the LOG converting portion 106 by a period (line delay)
during which a black character determining portion (not shown)
generates a control signal such as UCR, FILTER, and SEN by the
output from the input masking portion 105.
[0063] A masking UCR portion 108 extracts a black component signal
K from the image signal output from the line delay memory 107.
Furthermore, the masking UCR portion 108 performs matrix operation
for Y, M, C, and K, wherein color turbidities of the color
recording materials in the printing portion are corrected in the
portion 108, and color element image signals of, for example, eight
bits are output in the order of M, C, Y, and K every reading
operations in the reading portion 1R. Here, matrix coefficients
used for matrix operation are set by a not shown CPU 200.
[0064] Moreover, a .gamma. correction portion 109 performs density
correction of the image signal output from the masking UCR portion
108 in order to match the image signal with an ideal gradation
characteristics of the printing portion. An output filter (a
spatial filter processing portion) 110 performs edge emphasis
processing or smoothing processing for the image signal output from
the .gamma. correction portion 109, based on the control signal
from a CPU.
[0065] An LUT (an adjusting means) 111 matches the density of an
original image with that of an output image, and includes, for
example, RAM and the like. The translation table is set by a CPU. A
pulse width modulator (PWM) 112 outputs a pulse signal with a pulse
width corresponding to the level of the input image signal, and the
pulse signal is input to a laser driver 113 driving a semiconductor
laser (a laser light source). The scanner 3 forms an electrostatic
latent image by scanning exposure of the surface of the
photosensitive drum 1 by laser light, based on the image signal
input from the image reading means 21.
(Decision of a Recording Rate of Dark and Light Toners)
[0066] When it is decided that a sheet is corresponding to which of
the recording paper A, B, and C, "recording rates Rn and Rt" are
decided, which may be paraphrased as a mixing (allocation) rate of
dark toners and light toners adaptable to the sheet paper kinds,
that is, a toner dot forming rate, and the above decision is made
for both magenta and cyan.
[0067] FIG. 9 shows characteristic curves determining recording
rates of dark and light toner for dark toners denoted by a
solid-line curve, and light toners denoted by a dashed-line curve.
Based on a color component image signal Data of eight bits, which
is data for the cyan component and the magenta component, recording
rates Rn and Rt are decided for dark toners and light toners,
respectively. It is possible to interpret that the recording rate
represents a quantity of formed dots, a larger rate shows high
density, and a smaller rate shows low density.
[0068] When input gradation data is, for example, 100/255, it is
decided that a recording rate Rt for light toners is 255/255, and a
recording rate Rn for dark toners is 40/255. Here, the recording
rate is represented by an absolute value, assuming that 255
represents 100 percents.
[0069] Moreover, the following is found from FIG. 9. Defective
transfer is easily and noticeably caused by the effect of the
surface roughness of a sheet at a location in which the placed
toner quantity is large for light toners, and the location stands
out. On the other hand, defective transfer is hardly caused at a
location in which the placed toner quantity is small for dark
toners. Accordingly, in order to reproduce a low density portion,
it is preferable to raise the recording rate for dark toners. Then,
based on the color component image signal Data, the recording rates
Rn and Rt are changed according to the recording paper A, B, and
C.
[0070] When it is found from the result of the above-described
determination that the sheet 32 is corresponding to, for example,
the recording paper A, the recording rates for the light toners and
the dark toners are changed to be adaptable to the recording paper
A, as shown in FIG. 10. The reason is that, as the paper cellulose
on the surface is coarse, and a lot of cellulose shadows are
generated in the case of the recording paper A as shown in FIG. 3
and FIG. 4, there is a great difference between the bright
locations and the dark locations, and the difference between
highest pixel density Dmax and lowest pixel density Dmin becomes
larger. Similarly, the recording rates are set at a value between
those for the recording paper A and B when it is found from the
result of the above-described determination that the sheet 32 is
corresponding to the recording paper B.
[0071] Conditions in which the recording rates for the light toners
and the dark toners are changed as shown in FIG. 10 will be
described in detail. As shown in FIG. 10, a use quantity of dark
toners is increased as image data for a toner image to be formed is
increased. In the present example, the density of a toner image
with the lowest density among toner images formed by using dark
toners is changed according to the surface roughness of the sheet.
That is, a toner image with the lowest density among toner images
formed by using dark toners and light toners is changed by the
surface roughness of the sheet. When the sheet is corresponding to
the recording paper A as shown in FIG. 10, dark toners are used for
from image data 20 through 255. When the sheet is corresponding to
the recording paper B, dark toners are used for image data within a
range of 50 through 255. Moreover, when the sheet is corresponding
to the recording paper C, dark toners are used for image data
within a range of 75 through 255.
[0072] According to the present embodiment, the recording rates for
light toners and dark toners with regard to magenta and cyan are
changed according to kinds such as the surface roughness and the
like of the sheet 32 as described above, wherein the surface
roughness are represented by the recording paper A, B, and C.
Thereby, a high-quality image without image unevenness may be
formed according to the sheet kinds.
[0073] In the image forming apparatus according to the present
embodiment, a pattern generator (not shown) may be installed for
registration of gradation patterns, and a signal may be directly
transmitted to a pulse width modulator 62.
[0074] In the present example, the surface roughness of a sheet has
been detected by the sheet reading and detecting portion 123.
However, it is also possible to adjust the image controlling method
according to the surface roughness of a sheet to be used which is
input by a user through an operation panel 210 as shown in FIG.
11.
[0075] Moreover, the present example has described the device in
which, once a toner image formed on the photosensitive drum 1 is
transferred onto the intermediate transfer belt, the toner image is
transferred onto a sheet. However, the present invention may be
also applied to an image forming apparatus in which a toner image
formed on the photosensitive drum 1 is transferred directly to a
sheet.
[0076] (Second Embodiment) According to a second embodiment, a
content ratio between dark toners and light toners is changed
according to sheet kinds (surface roughness) in a similar manner to
that of the first embodiment, and, furthermore, the resolution in a
low density portion is reduced according to sheet kinds (surface
roughness) in an image forming apparatus having a similar
configuration to that according to the first embodiment.
[0077] Details will be described hereinafter, but only different
points in the configuration and in the control from those of the
first embodiment will be described. The reason is that the
configuration and operations of the main body are similar to those
of the first embodiment.
[0078] Usually, there are instabilities in electrostatic latent
images, and unstable factors generated by the above instabilities
in low density portions. Furthermore, there are unstable factors in
processing during which a toner image is transferred onto the sheet
32. In order to eliminate the above unstable factors for
improvement, it is effective for suppression of influences caused
by the surface state and the like of a sheet to reduce the
resolution in a low density portions.
[0079] FIG. 12A and FIG. 12B are a schematic view showing states in
which, when sheet paper kinds are expressed by the rugged state of
a sheet surface, that is, "surface roughness", transferring of a
toner image formed on the image bearing member such as the
photosensitive drum 1 onto a sheet is influenced by the surface
roughness of a sheet.
[0080] In FIG. 12A, a toner image before transferring is formed in
a uniform thin layer on the image bearing member. But, a
well-transferred state is not obtained after transferring because
electric discharge is generated in concave portions caused by the
surface roughness of the sheet at electrostatic transferring.
Thereby, image defectives are caused by transferring unevenness
(image unevenness) according to degrees of the surface roughness of
the sheet. On the other hand, as shown in FIG. 12B, an image is
formed with less influences by the surface roughness of the sheet
when a toner image before transferring is concentrated (the number
of lines is reduced) in a massive state on the image bearing
member. Thereby, there may be reproduced low density portions with
regularity schematically expressed in a checkered pattern, and
image defectives caused by transferring unevenness are
prevented.
[0081] When the kind of a sheet is decided according to the flow
diagram shown in FIG. 5, the pulse width modulator (resolution
changing means) 112 shown in the block diagram of FIG. 7 changes
the number of lines of a toner image of image data 100 or less
according to the kinds of a sheet (surface roughness), that is, the
resolution in the direction of the rotation axis of the
photosensitive drum 1 according to the kinds of a sheet (surface
roughness). In a low density portion of image data 100 or less,
resolution at use of a sheet corresponding to the recording paper A
with a rough surface is reduced in comparison with the number of
lines (resolution) at use of a sheet corresponding to the recording
paper C with a smooth surface). At this time, the number of lines
for an image exceeding image data 100 is 200 regardless of the kind
of a sheet.
[0082] The number of lines are changed as shown in Table 1. Here,
[lpi] in Table 1 is a number of lines per one inch.
TABLE-US-00001 TABLE 1 Sheet Kind Number of Lines [lpi] Recording
Material A 130 Recording Material B 170 Recording Material C
200
[0083] Though the image forming apparatuses according to several
embodiments of the present invention has been described above, the
invention is not limited to the above embodiments, and other
embodiments, various kinds of applications, modifications, and
combinations thereof may be applied without departing from the
scope of the invention.
[0084] It has been described in the embodiments that best image
adjustment is performed by adjusting recording rates of dark and
light toners using magenta toners and cyan toners, assuming that
the present invention is applied to a color-image forming
apparatus. But, similar advantages may be obtained even when light
toners are applied to gray-colored toners, and dark toners are
applied to black-colored toners.
[0085] This application claims the benefit of priority from the
prior Japanese Patent Application No. 2006-268291 filed on Sep. 29,
2006 the entire contents of which are incorporated by reference
herein.
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